Electronic system to detect objects in confined volume

ABSTRACT

A system to detect changes in the contents of a defined sensitive volume wherein a detector having an active resonant circuit is connected in an oscillator circuit to create an internal field in the defined sensitive volume, the frequency of the oscillator being shifted by a sudden change in the contents of the defined sensitive volume affecting the internal field. A receiver is coupled to the oscillator circuit and responsive to changes in the frequency of the oscillator to produce an output indicative thereof. Feedback means connected between the output of the receiver and the oscillator provide tracking between the oscillator and the receiver in the zero to low frequency range. Detect means are connected to the output of the receiver to produce a detect signal in response to a change in the contents of the defined sensitive volume affecting the internal field. The system also includes indication means connected to the output of the detect means activated in response to detect signals to provide an indication of a change in the contents of the defined sensitive volume affecting the internal field, and error prevention means connected to the detect means and the indication means operative to prevent activation of the indication means in the event erroneous detect signals are produced.

United States Patent 1191 Vosteen ELECTRONIC SYSTEM TO DETECT OBJECTS INCONFINED VOLUME Inventor: Robert E. Vosteen, 315 West Center St.,Medina, NY. 14103 Filed: Apr. 21, 1971 Appl. No.: 136,153

Primary Examiner-Harold I Pitts Attorney-lrons, Sears, Staas, Halsey &Santorelli [57] ABSTRACT A system to detect changes in the contents of adefined sensitive volume wherein a detector having an active resonantcircuit is connected in an oscillator circuit to create an internalfield in the defined sensitive volume, the frequency of the oscillatorbeing shifted by a sudden change'in the contents of the definedsensitive volume affecting the internal field. A receiver is coupled tothe oscillator circuit and responsive to changes in the frequency of theoscillator to produce an output indicative thereof, Feedback meansconnected between the output of the receiver and the oscillator providetracking between the oscillator and the receiver in the zero to lowfrequency range. Detect means are connected to the output of thereceiver to produce a detect signal in response to a change in thecontents of the defined sensitive volume affecting the internal field.The system also includes indication means connected to the output of thedetect means activated in response to detect signals to provide anindication of a change in the contents of the defined sensitive volumeaffecting the inter nal field, and error prevention means connected tothe detect means and the indication means operative to preventactivation of the indication means in the event erroneous detect signalsare produced.

8 Claims, 7 Drawing Figures PART DETECT SCHMITT j 5 NOISE THRESHOLDgLgfi i A R29 T I w R28 tiv'v W 0 He 4% 1253s 1 INPUT 24 L E 4 H6 $7 6'A 1 R25}? R22 0203 q 1 l R23 S SENSITIVITY O 4 V 10910 OUTPUTS Fm PARTDETECT ONE-Kick (1211;: R30 I 0 T3 LOGIC"ZERO"OUTPUT g V Losle'w'o uiJuly 3, 1973 v PATENTEIIJIII. a ma,

RElAYS INDICATORS DETECT 8: LOGIC CIRCUITS i FEEDBACK SIGNAL CONDITIONERRECE I VER DIRECTION OF MOVING PARTS EXIT DETECTOR OSCILLATOR FIG. 1

V ENTRY FIG. 3a

INVENTOR ROBERT E. VOSTEEN Irma, fem-r, Stem, BY fi/Mny i SwamiATTORNEYS PATENIEBJIIL am:

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(GROUND) PAIENIEum a ma SIGNAL INPUT DC COMMON m6 FROM RECEIVER AFCINTEGRATOR i a gm OUTPUT ELECTRONIC SYSTEM TO DETECT OBJECTS IN CONFINEDVOLUME BACKGROUND OF THE INVENTION Field of the Invention The inventionrelates to a detection system that reliably detects the sudden presenceor absence of an object in a defined sensitive volume. It has particularutility, for example, for attachment to automatic production machineryto verify part production and release before allowing the machinery toproceed to produce the next part. A punch press, for example, must ejectthe part produced before permitting production of the next part ,or thepress or die can be seriously damaged.

SUMMARY OF THE INVENTION Applicants invention provides a detector havinga high frequency oscillator with a resonant circuit that produces anelectromagnetic field. The resonant circuit may comprise, for example, atank circuit including an inductive sensor coil and having a naturalresonant frequency in the ll00 megacycle range. Any distortion of thefield produced by the tank circuit due to the presence of a foreignobject therein will influence the inductance and/or capacitance of thetank circuit and will thereby shift the resonant frequency of the tankcircuit.

The output of the oscillator is fed to an FM receiver tuned to producean output signal proportional in amplitude to the resonant frequencyshift of the tank circuit. The FM receiver may thereby detect whether ornot a foreign object is even momentarily within the defined sensitivevolume of the field produced by the tank circuit. Consequently, ifcorrection is made for incidental FM and tank frequency drift, even themomentary presence or absence of a foreign object within the definedsensitive volume may be detected.

The output spectrum of the FM receiver is optimized for signal-to-noiseratio of the object to be detected. This optimized signal is fed to anabsolute value circuit to insure a fixed polarity of output, independentof input polarity. The output of the absolute value circuit is connectedto a trigger circuit that produces output pulses which affect relayclosures and corresponding indications and/or alarms, depending uponwhether or not an object is within the defined sensitive volume. Otherfeatures of the invention include a fail-safe circuit operative in theevent of detector failure. Further, drift due to temperature shift,supply instability and detector contamination by oil and dirtaccumulation is compensated for by means of an integrator automaticfrequency control system which controls the average frequency of thedetector and holds it at a preset average frequency independent of theabove enumerated error introducing influences.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of thedetection system according to applicant's invention;

FIG. 2 is an electrical schematic diagram of the detector and oscillatorelements comprising applicant's invention;

FIG. 3 is a mechanical drawing of one particular type of sensor coilassemblage that may be employed in applicant's invention, wherein,

FIG. 3(a) is an end view of the assemblage,

FIG. 3(b) is a partial sectional view taken along section A-A, and

FIG. 3(a) is a side view of the assemblage;

FIG. 4 is a detailed electrical schematic diagram of the signal-to-noiseoptimizer, absolute value generator and AFC integrator circuits of theinvention;

FIG. 5 is an electrical schematic diagram of the circuits to which theabsolute value output of FIG. 4 is applied, and details particular logicand detector circuits which function to give an accurate indication ofeven the momentary presence of an object within the de fined sensitivevolume.

DETAILED DESCRIPTION OF THE INVENTION FIG. 2 is a schematic drawing ofthe oscillator circuit having a varying resonant frequency dependingupon the contents of the defined sensitive volume. The resonant tankcircuit of the oscillator comprises'inductance Ll which may typicallyconsist of a sensor coil having two or more turns, and produces aninternal electromagnetic field.

The sensor coil and one possible form of its associated mechanicalassemblage are shown in greater detail in FIG. 3. Sensor coil L1 isillustrated as comprising a two turn coil surrounded by conductiveshield 2 which confines the electromagnetic field and thereby definesthe sensitive volume 4. Sensor coil L1 is cast in dielectric foammaterial 3 to minimize any relative movement between the electrostaticshield 2 and the sensor coil L1. The defined sensitive volume 4 isslightly smaller in cross section than the inner dimensions of sensorcoil L1. FIG. 3 illustrates the defined sensitive volume as beingcylindrical. In particular it should be apparent that the definedsensitive volume 4 can take many forms in adapting to (l) the availablespace and (2) the shape and size of the object to be detected. Othertypical forms may comprise squares and rectangles, for example. Theconductive shield 2 defines entry and exit apertures to create a voidbetween the limits of the shield and the inner portion of the sensorcoil L1. The object being detected passes through this void and causes achange in the internal field and thus the oscillator frequency. Theremaining structure illustrated in FIG. 3 comprises metallic end plates5 which also function as conductive shields and associated end plateclam ping rods 6. The detector may consist of a detector circuit board 7surrounded by detector circuit shield .box 8. The dielectric foam 3 ishoused in dielectric rigid plastic abrasion resistant cylinder 9.Mounting brackets 10 are also shown in FIG. 1.

The sensor coil assemblage thus comprises a multiturn sensor coilsurrounded by an electrostatic shield to confine the electromagneticfield produced by the coil, with the electrostatic shield definingapertures to permit the entry and exit of the object to be detected. Theoscillator circuitry should be located in close proximity to the sensorcoil to permit the use of the coil as a component of the resonantoscillator. The elements comprising the assemblage should be rigidlymounted with respect to each other and their surroundings to minimizeincidental frequency modulation attributable to shock and vibration, andthe dielectric foam in which the sensor coil is encased establishesfixed rigid geometry between the electrostatic shield and the sensorcoil.

FIG. 2 shows the connection of sensor coil L1 in parallel with capacitorC8. The physical capacitance of the resonant tank circuit of theoscillator comprises dual varactor diode D1 and capacitor C8. Theanode-toanode capacitance of diode D1 is adjusted by the reverse biasedDC voltage established between the varactor anode and cathode asdiscussed more fully hereafter, and thus permits remote tuning of thedetector by means of a DC voltage. This provides a convenient techniquein instances where the detector is relatively inaccessible. PNPtransistor T1 functions in conjunction with the two terminal resonantcircuit as an oscillator. It is connected in the grounded baseconfiguration with positive feedback from the emitter of transistor T1to the collector through the connection of capacitor C7 therebetween.Resisters R8 and R9 are connected between the positive supply terminaland DC common with the series connection of resistors R8 and R9 beingconnected to the base of transistor T1. Capacitor C11 is connectedbetween the base of transistor T1 and DC common.

The described connection of resistors R8 and R9 bias the base oftransistor T1 and capacitor C11 bypasses the base of transistor T1 toground when it is operating in the grounded base mode.

Resistors R6 and R7 are connected between the positive supply terminaland the emitter of transistor T1, and the series connection of resistorsR6 and R7 is connected through capacitor C to the output coaxial cablewhich feeds the receiver. Resistor R10 is connected between the outputcoaxial cable and DC common. A pulsed current at the resonant frequencywill exist at the emitter of transistor T1 and therefore in resistorsR7, R6 and R10. Capacitor C10 is a coupling capacitor and thecombination of resistors R10 and R6 constitutes a low impedance sourceto drive the output coaxial cable.

The pulsed voltage that thus feeds the coaxial output is the inputsource for the receiver. Inasmuch as it is a pulsed voltage, it is richin harmonics and the receiver can consequently be tuned to a harmonic ofthe resonant frequency of the oscillator. This, for example, enables theuse of a receiver operating in the vicinity of 100 MHz while theoscillator can operate at any integral subharmonic between 10 MHz and.100 Mhz of the frequency to which the receiver is tuned. Because thereceiver sensitivity must be high to insure a high noise output in theabsence of a detector input signal, indicating the failure of thedetector, there is no problem in producing an adequate signal level at aharmonic of the detector output.

- It was previously discussed that it is desirable to tune the detectorover a relatively wide range by the use of a remote DC signal. This DCsignal must contain an extremely low level of ripple and noise tominimize incidental FM in the detector output which might in turn maskthe desired signal. The low noise variable DC signal is fed throughresistors R2, R4 and R5 to the varactor diode D1 and particularly to thecathode elements thereof. Resistors R1 and R3 are connected betweenterminal B and the series connection of resistors R4 and R5. Theresistance values of resistors R1 and R3 are higher than those ofresistors R2 and R4 and present a parallel path of lesser sensitivity tothe varactor diode D1 through resistor R5 for the purpose of automaticfrequency control of the detector. Capacitors C1, C2, C3, C4, C5, C6 andC9 function as RF bypass capacitors. The described detector is shown asoperating from a single positive supply but it should be apparent tothose in the art that a single negative supply can be employed inconjunction with an NPN transistor.

The receiver is shown connected to the coaxial cable output FIG. 2 andmust be capable of being tuned to the fundamental or a harmonic of thedetector freqiiency. Additionally, the receiver should have a directcoupled output, the output of the discriminator or ratio detector of anFM receiver being satisfactory. The amplitude limiting characteristic ofthe receiver should be good and the receiver should also have goodsensitivity because it must provide a high level of noise output in theabsence of aninput signal indicative of the detector failure.Additionally, the receiver should have a DC output characteristic whichincreases with increasing input frequency, a coaxial input to minimizestray signal pick up, and an extremely low level of incidental FM.

FIG. 4 shows the signal conditioning circuit portions of the smallobject or part detector with the output of the receiver feeding thesignal-to-noise optimizer. Amplifier A1 is connected as a DC amplifierwith a noninverting gain which may be typically on the order of throughthe feedback resistive network comprising resistors R14 and R15.Resistor R14 is shunted by variable capacitor C14, the combinationhaving a time constant (T1) equal to (R14 X C14) which determines thehigh frequency roll-off of amplifier A1. The capacitance of variablecapacitor C14 is selected to optimize the high frequency-to-noise ratioof the output signal of amplifier A1.

Resistor R16 is connected between the output of the receiver and theinput of amplifier A1 and capacitor C15 is connected between the seriesconnection of resistor R16 and said amplifier A1 input and DC common.The relative values of resistor R16 and capacitor C15 are selected toprovide additional high frequency noise suppression.

Variable capacitor C16 is connected between the output of amplifier A1and the input to the absolute value generator and functions as the inputcoupling capacitor to the absolute value generator. Capacitor C16determines the low frequency roll-off of the input to the absolute valuegenerator and the value of its capacitance is selected to optimize thelow frequency-to-noise ratio and thereby minimize the influence of lowfrequency vibration effects to the small parts detector.

In resume, the function of amplifier Al operating as a signal-to-noiseoptimizer is to control the low and high frequency roll-off feeding theabsolute value-generator. The latter is in essence an ultra-linearfullwave rectifier and is a well-known circuit. Consequently, detailedexplanation of this circuit is not provided herein. The absolute valuegenerator functions to generate a DC output signal equal in amplitude tothe alternating voltage appearing at the common connection of resistorsR17 and R18 and capacitor C16. The output of the absolute valuegenerator is always negative in polarity but of identical magnitude tothe input signal, regardless of the polarity of the input signal.Inasmuch as the absolute value generator generates an output of a givenpolarity independent of the input polarity, a sudden increase ordecrease in detector frequency of equal magnitude will generate anidentical output signal from amplifier A3.

In the event of detector failure it is desirable to provide an alarm soas not to inaccurately record the presence of an object within thedefined sensitive area if in fact an object is not present. It wasdiscussed above that the receiver should have very good sensitivity withthe result that a high noise level is at the receiver output when theoscillator is disabled. This high noise level will be demodulated by theabsolute value generator circuit whose input is capacitor coupled bycapacitor C16, and which therefore has no DC component of input signal.The presence of a large DC component at the amplifier A3 output of theabsolute value generator would thus be evidence of detector failure anda suitable alarm circuit can be connected to the output of the absolutevalue generator to give an indication of detector failure. If this DCcomponent were to be ignored the high noise level could otherwise beconstrued as a valid detect signal.

The small parts or objects detector according to the invention alsocomprises an automatic frequency control (AFC) integrator circuit toprovide tracking of low frequency or DC components of the oscillatorfrequency deviation to the receiver. Typically, the low frequencycomponents would be in the infrasonic range. Inasmuch as the output fromthe receiver normally contains no DC component, the average DC output ofthe receiver is zero. In the event the detector shifts in frequency forany reason such as a gradual accumulation of dirt or oil, a lowfrequency or DC component will have time to develop at the receiveroutput. This low frequency or DC component as shown in FIG. 3 will befed to the input of the AFC integrator circuit and particularly throughresistor R16 to the output of amplifier A4 which comprises aconventional analog integrator comprising an operational amplifier withcapacitor C17 connected as the feedback capacitor. The output ofamplifier A4 is a negative integral of the receiver output. This is theAFC output and feeds the detector AFC input as shown in FIG. 1.

Equilibrium or zero output from the receiver will be reestablished whenthe AFC integrator output reaches that value which reestablishes thecorrect output frequency from the detector, to wit, the frequency towhich the receiver is tuned. The AFC action described is thus theopposite of a conventional FM receiver in that the receiver tuningremains at a fixed frequency and the AFC functions to shift thefrequency of the RF source, the detector in this case, to reestablishthe source frequency at the desired frequency. This procedure ispreferable over shifting the frequency of the receiver local oscillatorbecause it permits the system to operate at a fixed unused frequency andthus permits possible interference due to the detector shifting infrequency and possibly crossing a frequency on which another signalexists. If the interfering signal were stronger, although this is notlikely, the receiver would lock into the wrong signal andthe systemwould cease to function.

It was previously discussed inrelation to FIG. 2 that the detector maybe remotely tuned by means of a second DC source feeding varactor diodeD1. In order to accomplish this, a dual contact push-button switch PB isconnected as shown in FIG. 3. A tuning meter V is also shown in FIG. 3and is normally connected to the output of the AFC integrator throughnormally closed contact PB] through ground. Thus, meter V normallycontinuously meters the AFC integrator output signal feeding thedetector. In the event this signal approaches the linear limits of theintegrators capability, the detector should be retuned to establish anormal integrated voltage nearer to zero volts out. This retuning isaccomplished by first depressing switch PB which (1) shorts the AFCintegrator by affecting a short circuit through contact PB2 and (2)connects the meter to the amplified receiver output of amplifier A1 ofthe signaltonoise optimizer. The detector can now be retuned to afrequency producing an output from amplifier A1 near zero volts, andthus nearer the correct frequency, at which time the push-bottom switchPB can be released to cause the AFC integrator to correct for anyresidual mistuning.

The output of the absolute value generator is applied to two Schmitttrigger circpits as shown in FIG. 5, namely, amplifier A5 whichcomprises the object or part detect Schmitt trigger circuit andamplifier A6 which comprises the detector fault Schmitt trigger circuit.Amplifiers A5 and A6 are connected in conventional operational amplifierform and in conjunction with other elements comprise conventionalSchmitt trigger circuits. Hence, detailed discussion thereof is notincluded herein. When a negative going output is applied to amplifier A5through resistor R21 fed from the absolute value output and it crossesthe trip level, which may for example be negative but near zero volts,the part detect Schmitt output jumps to a positive voltage (which may beapproximately 9 volts for example). When the signal applied to amplifierA5 crosses back going positively, the Schmitt output jumps to a negativevoltage. (Which may be approximately 0.6 volts for example.) Positivefeedback is provided from the output of amplifier A6 through resistorsR22 and R23 to produce hysteresis to reduce the chance of triggering dueto a noisy signal.

An adjustable positive DC signal labeled sensitivity is applied throughresistor R24 to the input of operational amplifier A5. The amplitude ofthe positive DC signal may be varied by varying the setting of thepotentiometer comprising resistor R25 connected between the positivesupply terminal and DC common. By adjusting the sensitivity control, theminimum negative signal necessary for triggering the part detect Schmittcircuit can be established. The part detect Schmitt trigger circuit willthen ignore all input signals having a peak DC negative voltage lessthan this preset threshold.

It was previously discussed that a steady DC component is developed atthe absolute value output because of the high noise content at thereceiver output in the event of detector failure. The detector faultSchmitt trigger circuit is very similar to the already described partdetect Schmitt trigger circuit except that its signal input is filteredby the low pass filter network comprising resistor R26 and capacitor C20connected between the absolute value input and DC common. Resistor R27,connected between the series connection of resistor R26 and capacitorC20, and the input terminal of operational amplifier A6, serves the samefunction as resistor R21 connected between the absolute value input andthe input of operational amplifier A5. Consequently, the detector faultSchmitt trigger circuit will not trigger except in the event of asustained DC component, which is the circumstance existing in the eventof detector failure. This threshold of operation for the detector faultSchmitt trigger circuit can be preset by the noise threshold control,which comprises resistor R28 connected as a potentiometer between theabsolute value output and DC common, and its connection through resistorR29 to the input of operational amplifier A6. The noise thresholdcircuit thus functions similarly to the sensitivity control circuit.

The part detect Schmitt trigger circuit feeds the part detect one-kickcircuit by the connection to the latter of the output of operationalamplifier A5. A positive pulse from the output of the part detectSchmitt trigger circuit to the part detect one-kick circuit causes thelatter to generate a single positive pulse of fixed amplitude and widthat its output for each part detect Schmitt trigger output, regardless ofthe duration of the part detect Schmitt output. The part detect one-kickcircuit is a conventional monostable multivibrator whose positive inputpulse is fed through capacitor C21 and whose positive output isdeveloped at the collector of transistor T2. Because it is aconventional monostable multivibrator circuit, the other elementscomprising the part detect one-kick circuit will not be describedbecause operation thereof is apparent to one in the art.

Two logic circuits are provided, one being inverted with respect to theother. In the event a part is detected, transistor T3 which inverts thepart detect onekick output goes from a logic 1 to a logic state.Transistor T3 also feeds transistor T4, a second inverter whichtherefore in the event of a detected part goes from a logic 0 to a logic1 output.

The output of the part detect one-kick circuit also feeds the detectrelay through the relay driver comprising transistor T5. Thus when apart is detected within the defined sensitive volume, transistor T5 isdriven to the conducting state for a fixed time interval through thepart detect one-kick circuit and relay K1 is energized for this fixedinterval to activate the detect relay output. Correspondingly,transistor T5 also drives transistor T6 which functions as a constantcurrent driver for detect light L1 which provides a visual indication ofa detect signal.

In the event of a control system failure in the receiver, signalconditioning, power failure, etc., a detect output will not exist if thesystem is fail-safe. However, in the event of detector failure, unlessprecautions are taken, the noise at the receiver output could beconstrued as a great proliferation of valid signals by the circuitry. Toprevent this malfunction, the detector fault Schmitt trigger circuit istriggered as previously described. This Schmitt circuit drivestransistor T3, a relay driver,'which activates detector fault relay K2and thereby an external alarm through the external alarm contacts. Theconnection to the detector fault light L2 is similarly completed throughactivation of relay K2.

To simultaneously shut down the automatic system in the event of adetector fault, the detector fault Schmitt trigger circuit drivestransistors T8 and T9, the bases of these transistors being connectedthrough resistors R30 and R31, respectively, to the output of thedetector fault Schmitt trigger circuit. In the event of a detectorfault, activated transistor T8 shunts the input to the logic circuitsand prevents a detect signal output. Similarly, activated transistor T9shunts the input to the detect relay driver, thus preventing actuationof the detect relay.

The system according to the invention is capable of reliably detecting apart (I) whose volume is less than l0 parts per million of the definedsensitive volume, (2) which interjects any path through the definedsensitive volume, and (3) which moves through the defined sensitivevolume at very high speed. It should be apparent that the problemsassociated with detection of relatively larger objects or objects movingat a relatively slower speed is easier. In addition to determining thepresence or absence of an object from the defined sen- I may be made toapplicants invention without departing from the scope thereof. Forexample, while the specification and drawings show tracking of theoscillator to the receiver, the reverse, that is tracking of thereceiver to the oscillator, may also be provided.

I claim:

1. A system to detect changes in the contents of a defined sensitivevolume comprising:

a detector having an active resonant circuit connected in an oscillatorcircuit to create an internal field in the defined sensitive volume, thefrequency of the oscillator being shifted by a sudden change in thecontents of the defined sensitive volume affecting the internal field,

a receiver coupled to the oscillator circuit and responsive to changesin the frequency of the oscillator to produce an output indicativethereof,

feedback means connected between the output of the receiver and theoscillator to provide tracking between the oscillator and the receiverin the zero to low frequency range,

detect means connected to the output of the receiver to produce a detectsignal in response to a change in the contents of the defined sensitivevolume affecting the internal field,

indication means connected to the output of the detect means activatedin response to detect signals to provide an indication of a change inthe contents of the defined sensitive volume affecting the internalfield,

error prevention means connected to the detect means and the indicationmeans operative to prevent activation of the indication means in theevent erroneous detect signals are produced.

2. A system to detect changes as recited in claim 1 wherein there istracking of the oscillator to the receiver.

3. A system to detect changes as recited in claim 2 wherein the feedbackmeans comprise an integrator to reduce the average output error of thereceiver to substantially zero.

4. A system to detect changes in the contents of a defined sensitivevolume comprising:

a detector having an active resonant circuit connected in an oscillatorcircuit to create an internal field in the defined sensitive volume, thefrequency of the oscillator being shifted by a sudden change in thecontents of the defined sensitive volume affecting the internal field,

a receiver coupled to the oscillator circuit and responsive to changesin the frequency of the oscillator to produce an output indicativethereof,

signal conditioning means comprising a signal-tonoise optimizerconnected to the output of the receiver to optimize the signal-to-noiseratio of the receiver output,

feedback means connected between the signal conditioning means and theoscillator to provide tracking between the oscillator and the receiverin the zero to low frequency range,

detect means connected to the output of the signal conditioning means toproduce a detect signal in response to a change in the contents of thedefined sensitive volume affecting the internal field,

indication means connected to the output of the detect means activatedin response to detect signals to provide an indication of a change inthe contents of the defined sensitive volume affecting the internalfield,

error prevention means connected to the detect means and the indicationmeans operative to prevent activation of the indication means in theevent erroneous detect signals are produced.

5. A system to detect changes as recited in claim 4 wherein there istracking of the oscillator to the receiver.

6. A system to detect changes as recited in claim 5 wherein the feedbackmeans comprise an integrator to reduce the average output error of thereceiver to substantially zero.

7. A system to detect changes as recited in claim 4 wherein the signalconditioning means further comprise an absolute value generatorconnected between the signal-to-noise optimizer and the detect means toproduce a single polarity output in response to an input of eitherpolarity.

8. A system to detect changes as recited in claim 7 wherein the signalconditioning means is connected to produce a DC component in the eventof detector failure and the error prevention means comprise a detectorfailure circuit operative in response to said DC component to inactivatethe indication means.

1. A system to detect changes in the contents of a defined sensitivevolume comprising: a detector having an active resonant circuitconnected in an oscillator circuit to create an internal field in thedefined sensitive volume, the frequency of the oscillator being shiftedby a sudden change in the contents of the defined sensitive volumeaffecting the internal field, a receiver coupled to the oscillatorcircuit and responsive to changes in the frequency of the oscillator toproduce an output indicative thereof, feedback means connected betweenthe output of the receiver and the oscillator to provide trackingbetween the oscillator and the receiver in the zero to low frequencyrange, detect means connected to the ouTput of the receiver to produce adetect signal in response to a change in the contents of the definedsensitive volume affecting the internal field, indication meansconnected to the output of the detect means activated in response todetect signals to provide an indication of a change in the contents ofthe defined sensitive volume affecting the internal field, errorprevention means connected to the detect means and the indication meansoperative to prevent activation of the indication means in the eventerroneous detect signals are produced.
 2. A system to detect changes asrecited in claim 1 wherein there is tracking of the oscillator to thereceiver.
 3. A system to detect changes as recited in claim 2 whereinthe feedback means comprise an integrator to reduce the average outputerror of the receiver to substantially zero.
 4. A system to detectchanges in the contents of a defined sensitive volume comprising: adetector having an active resonant circuit connected in an oscillatorcircuit to create an internal field in the defined sensitive volume, thefrequency of the oscillator being shifted by a sudden change in thecontents of the defined sensitive volume affecting the internal field, areceiver coupled to the oscillator circuit and responsive to changes inthe frequency of the oscillator to produce an output indicative thereof,signal conditioning means comprising a signal-to-noise optimizerconnected to the output of the receiver to optimize the signal-to-noiseratio of the receiver output, feedback means connected between thesignal conditioning means and the oscillator to provide tracking betweenthe oscillator and the receiver in the zero to low frequency range,detect means connected to the output of the signal conditioning means toproduce a detect signal in response to a change in the contents of thedefined sensitive volume affecting the internal field, indication meansconnected to the output of the detect means activated in response todetect signals to provide an indication of a change in the contents ofthe defined sensitive volume affecting the internal field, errorprevention means connected to the detect means and the indication meansoperative to prevent activation of the indication means in the eventerroneous detect signals are produced.
 5. A system to detect changes asrecited in claim 4 wherein there is tracking of the oscillator to thereceiver.
 6. A system to detect changes as recited in claim 5 whereinthe feedback means comprise an integrator to reduce the average outputerror of the receiver to substantially zero.
 7. A system to detectchanges as recited in claim 4 wherein the signal conditioning meansfurther comprise an absolute value generator connected between thesignal-to-noise optimizer and the detect means to produce a singlepolarity output in response to an input of either polarity.
 8. A systemto detect changes as recited in claim 7 wherein the signal conditioningmeans is connected to produce a DC component in the event of detectorfailure and the error prevention means comprise a detector failurecircuit operative in response to said DC component to inactivate theindication means.